1. Field of the Invention
The present invention relates to a semiconductor device having a circuit including a thin film transistor (hereinafter referred to as a TFT) and a manufacturing method thereof. For example, the present invention relates to an electronic device provided with, as a component, an electro-optical device typified by a liquid crystal display panel or a light-emitting display device including an organic light-emitting element.
Note that, in this specification, a semiconductor device refers to any device which can function by using semiconductor characteristics. An electro-optical device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) using a semiconductor thin film (with a thickness of about several nm to several hundreds nm) formed over a substrate having an insulating surface has been attracted. Thin film transistors have been widely applied to electronic devices such as ICs and electro-optical devices, and their development especially as switching elements for an image display device has been accelerated.
A thin film transistor using an amorphous semiconductor film, a thin film transistor using a polycrystalline semiconductor film, or the like is used as switching elements in image display devices.
In the case of the thin film transistor using an amorphous semiconductor film, an amorphous semiconductor film such as a hydrogenated amorphous silicon film is used; therefore, there is limitation on the process temperature, and heating at a temperature of 400° C. or more at which hydrogen is detached from the film, intense laser light irradiation by which surface roughness occurs due to hydrogen in the film, and the like are not performed.
Further, as a method for forming a polycrystalline semiconductor film, the following technique is known: dehydrogenation treatment of reducing the hydrogen concentration in an amorphous silicon film is preformed in advance in order to prevent occurrence of surface roughness; a pulsed excimer laser beam is shaped into a linear shape with an optical system; and the dehydrogenated amorphous silicon film is scanned and irradiated with the linear beam for crystallization.
A thin film transistor using a polycrystalline semiconductor film has an advantage in that mobility is higher than that of a thin film transistor using an amorphous semiconductor film by two or more digits, and a pixel portion and a peripheral driver circuit of a display device can be formed over the same substrate. However, the thin film transistor using a polycrystalline semiconductor film requires a more complicated process than the thin film transistor using an amorphous semiconductor film because of crystallization of the semiconductor film; thus, there are problems such as reduction in yield and increase in cost.
The present applicant has disclosed an FET (field effect transistor) in which a channel formation region is formed of a semiconductor having a mixture of a crystalline structure and a noncrystalline structure in Patent Document 1 (U.S. Pat. No. 5,591,987).
In addition, as switching elements in image display devices, a thin film transistor using a microcrystalline semiconductor film has been used (Patent Document 2: Japanese Published Patent Application No. H4-242724 and Patent Document 3: Japanese Published Patent Application No. 2005-49832).
As a conventional method for manufacturing a thin film transistor, a technique is known in which an amorphous silicon film is formed over a gate insulating film, a metal film is formed over the amorphous silicon film, and the metal film is irradiated with a diode laser so that the amorphous silicon film is changed into a microcrystalline silicon film (Non-Patent Document 1: SID'07 Digest, pp. 1370-1373, 2007, Toshiaki Arai, et al.). According to this method, the metal film formed over the amorphous silicon film is to convert light energy of the diode laser into thermal energy, and the metal film needs to be removed later for completion of a thin film transistor. That is, this is the method in which the amorphous silicon film is heated only by conduction heating from the metal film to form the microcrystalline silicon film.